Process for Production of Compacted Graphite Iron

ABSTRACT

A process for production of compacted graphite iron using in-mould addition of a magnesium alloy is disclosed. The process is characterised by a step of pre-treating the base iron in a ladle or in a furnace with an alloy containing cerium and performing a structure forming treatment in a reaction chamber in the mould using an alloy containing magnesium and lanthanum.

FIELD OF THE INVENTION

The invention relates to a process for production of cast iron with astructure predominantly consisting of compacted graphite shapes. Theprocess is based on a combination of pre-treatment of a base iron and afinal treatment in the mould.

BACKGROUND ART

Compacted graphite iron is a cast iron alloy with a graphite structurebetween flake type graphite shapes and spherical shapes. The graphiteshape is determined by the conditions in the liquid iron during thesolidification. Treatment of a base iron, preferably with a carbonequivalent between 4.0 and 4.4 and with a sulphur content below 0.02%,with a ferrosilicon alloy containing 4-10% magnesium can be used toachieve the compacted graphite structure. The magnesium content must bekept within very narrow limits usually within +/− 0.003% and with alevel of about 0.008 to 0.015% depending on conditions of the base ironand the cooling rate in the casting to be produced. As used hereinpercent refers to percent by weight. The treatment with magnesium isusually made in a ladle. Magnesium boils at 1090° C. and since thetemperature of the iron usually is higher than 1400° C. during thetreatment some of the magnesium therefore vanishes as vapour, and somecombines with the sulphur, oxygen and nitrogen in the iron. Duringholding of the iron before pouring further reduction of the activemagnesium content occurs. This gradual reduction of active magnesium iscalled fading.

In order to avoid these problems the magnesium treatment can be madeinside each mould. That technology known as in-mouldtreatment/technology is well-known for production of ductile iron. Aspecial version of the technology as described in WO 01/54844 A1, issuitable for production of compacted graphite iron. The in-mouldtechnology is based on placing the magnesium alloy in a chamber in thegating system in the mould. During pouring, the iron flows into thechamber and gradually dissolves the alloy. The treated metal then fillsthe casting cavity. The problems with fading of magnesium are eliminatedwhen using this process.

One problem is that the sulphur level in the base iron often varies.Therefore the magnesium level must be adjusted. However, with thein-mould treatment this is practically not possible as the treatmentchamber is the same in each mould. Another problem is that treatmentwith magnesium makes the structure sensitive to variation in coolingrate. With a high cooling rate e.g. in thin sections of the casting thegraphite shapes tend to be more spherical. With long cooling rates i.e.in thick sections the graphite will precipitate as flakes.

It is known that treatment alloys containing both magnesium and ceriumreduce these problems. However high levels of cerium can increase therisk for certain casting defects such as formation of primary carbidesand shrinkages.

DESCRIPTION OF THE INVENTION

It is an object of the present invention to solve these problems.

The invention concerns a process for production of compacted graphiteiron using in-mould addition of a magnesium alloy in accordance withclaim 1. Preferred embodiments are defined in the dependent claims.

The amount of cerium is adjusted in relation to the sulphur content inthe base iron. The cerium level should be adjusted according to theformula:

%Cerium=(%Sulphur−0.006)* 2.9+A.

The value for A varies preferably between 0.01 and 0.03 depending on theconfiguration of the casting i.e. variation in section dimensions andcasting modulus. As cerium has a very high boiling point (3470° C.) anda high density (6.14 g/cm³) it does not show any fading effect. Byadding cerium to the base iron it can be properly dissolved and lessmagnesium alloy has to be added in the reaction chamber in the mould ascerium also has a structure forming effect.

The treatment alloy preferably contains 3-6% magnesium and 0.5-1.5%lanthanum. Lanthanum has a favourable effect in reducing defects such ascarbides and shrinkages in the casting. The effect on shrinkages ishighest just after treatment and therefore it is optimal to add thelanthanum as late as possible.

The alloys used can have various compositions since the paramountfeature is the overall fraction of active metal. However, examples ofcompositions for commercially available alloys include:

For the magnesium alloy: 48% Fe, 45% Si, 5% Mg, 1.0% A1, 0.5% La and0.5% Ca, and

for the Cerium alloy: 65% Fe, 25% Ce, 7% La, and a balance of other rareearth elements.

According to a preferred embodiment of the invention cerium is added tothe oven or the ladle (and not as a part of the magnesium alloy) andmagnesium is added to the mould.

With the proposed process the magnesium addition can be reduced with atleast 30% compared to a normal treatment in-the-mould without thepre-conditioning.

The reduced magnesium level in the castings has also the advantage thatcasting defects such as dross and micro-shrinkage are minimized.

1. A process for production of compacted graphite iron using in-mouldaddition of a magnesium alloy comprising pre-treating the base iron in aladle or in a furnace with an alloy containing cerium and performing astructure forming treatment in a reaction chamber in the mould using analloy containing magnesium and lanthanum.
 2. The process of claim 1,wherein the base iron is pretreated with an alloy containing cerium inorder to reach cerium levels between 0.008 and 0.025% and the iron isfurther treated in the casting mould using an alloy containing 3-6%magnesium and 0.5-1.5% lanthanum.
 3. The process of claim 1, wherein theminimum percentage of cerium in the base iron is estimated as (%S-0.006)*2.9+0.01), where S is the sulphur content in the iron before theaddition of cerium.
 4. The process of claim 2, wherein the minimumpercentage of cerium in the base iron is estimated as (%S-0.006)*2.9+0.01), where S is the sulphur content in the iron before theaddition of cerium.